"Well, I'll be horn-swaggled! Never could have imagined a combination rangefinder/reflex camera. My question is: why?"
When these cameras came out, the market was dominated by rangefinders. But as with all these constructions, macro work was not possible.
Alpa used this viewfinder/focusing combination to position themselves (and with the superior build quality, of course). The rangefinder for "usual" work, the mirror finder for macro. This is also the reason why the cameras up to model 3 (4?) had the vertical finders, quite convenient when the camera is pointed downwards. Later on the Kern built 45 degree viewfinder was introduced, which allowed easier macro focusing when working freehand (for flowers etcetc.)
Add on: the famous Kern Macro Switar was also used in this context, and f1.8 was (and is) quite uncommon for macro lenses, made the construction very expensive.
The later camera series wear "Alpa Alnea" badges, Alnea stands for "all near". Got it?